This invention relates to cellular communications systems and particularly to group handover procedures for reducing congestion in a cellular network.
Cellular radio telecommunications networks generally include mobile services switching centres (MSC) coupled to a public switched telephone network (PSTN), base transceiver stations (BTS) and radio telephone subscriber units often referred to as mobile stations.
Each of a plurality of base transceiver stations generally defines a geographic region or cell proximate to the BTS to produce coverage areas. Cell sizes range typically from 200 metres in diameter in urban areas to 60 kilometres in rural areas. Each BTS comprises the radio frequency components and the antenna for communicating with the mobile stations. The communication link from a BTS to a mobile station is called the downlink. Conversely, the communication link from the mobile station to the BTS is referred to as the uplink. Usually, several base transceiver stations are under the control of a base station controller (BSC) which in turn communicates directly by a land line or microwave link with an MSC. Several base station controllers may report to one MSC.
Multiple access techniques permit the simultaneous transmissions from several mobile stations to a single BTS. The GSM system (global system for mobile communications) uses time division multiple access (TDMA), in which a communications channel consists of a time slot in a periodic train of time intervals over the same frequency. Each mobile station is allocated one specific time slot for communication with a BTS in a repeating time frame. Another type of multiple access technique, and one proposed for the third generation universal mobile telecommunication system (UMTS) is known as Code Division Multiple Access (CDMA) which employs spread spectrum signaling. Individual users in the CDMA communications network use the same RF carrier frequency, but are separated by the use of individual spreading codes. Hence, multiple communications channels are allocated using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a mobile station. Further schemes involve combinations of both TDMA and CDMA.
By means of the aforementioned network components, a cellular network is able to (amongst other things) monitor the number of calls made by a mobile station, control the handover of the communications link from the mobile station to the base transceiver stations as it moves from one cell to another, calculate the round trip time (often referred to as the timing advance) of the communications link between the mobile station and the BTS and track a mobile station""s location within the network by means of a location update sequence.
As the mobile station moves from one cell to the next, in a GSM network, for example, the communication link is transferred from its current BTS to a neighbouring BTS using a procedure known as handover or handoff. The need for handover is usually determined on the basis of one or more criteria. Commonly used criteria are received signal strength indication (RSSI) of the mobile station at the BTS, the distance from the current BTS as determined by the round trip time for signals to and from the mobile station and the bit error rate (BER).
There is a variety of methods by which a handover process can be initiated. Firstly, processes for initiating handover may make measurements on the uplink and downlink between the mobile station and the BTS and initiate handover when received signal levels, received quality levels or the timing advance (based on the round trip time) values separately exceed thresholds. Alternatively, processes for initiating handover may compare measurements of received signal strength with received quality to deduce that bad quality may be the result of interference. Further, processes for initiating handover may additionally make measurements from a number of neighbour cells and initiate handover based on the relative level of the signal received from the serving cell and from the neighbour cells. There is a finite limit to the number of mobile stations that a single BTS can support at any given time. Sometimes the demand can exceed this limit and the cell supported by the BTS then becomes congested.
A handover can also be initiated even when the mobile station is not moving between cells. This type of handover is referred to as a traffic handover and occurs when a serving cell is congested and a neighbouring cell is not. If the neighbour cell""s coverage overlaps with the serving cell""s, then handing over some individual calls from the congested cell to a less congested cell improves the situation temporarily in the congested one.
In a third scenario, a mobile station may be moving out of a cell controlled by a TDMA-operated network into a cell controlled by a CDMA-operated network, thereby requiring handover between two different multiple-access systems. One example of an arrangement for handing over an individual call between two such systems is described in the Applicant""s co-pending Application WO-A-9623369.
If many mobile stations have to perform a handover during a short period of time, in order to de-congest an overloaded cell, for example, using one of the known aforementioned methods has a disadvantage of requiring a large signaling overhead. Further, in order not to degrade communications quality, the handover processes should be time-offset, therefore, the entire process takes a significant amount of time to complete. In the particular case of simultaneous multiple handovers from a UMTS to GSM system, many users would need to switch to a slotted mode of operation, consequently needing to increase their transmit power, thereby increasing battery power consumption and their contribution to interference to other users.
This invention aims to provide a mechanism for handing over a group of calls in progress so that congestion can be relieved in a given cell.
The invention further aims to facilitate handover between cells operating within the same network, e.g. GSM or between cells operating within different networks e.g. GSM and UMTS.
The invention also aims to facilitate a handover mechanism that can be implemented whether an individual mobile station or group of mobile stations is in motion or not.
The invention also aims to provide a group handover mechanism in which the signaling overhead is kept to a minimum.
Accordingly, the present invention consists of a radio telecommunications system including a base station serving a cell and for communicating with a plurality of mobile stations located within the cell, in which the system further includes;
means for monitoring a characteristic of transmissions between the base station and each mobile station,
means for clustering into a group those mobile stations whose monitored characteristic meets a pre-determined first criterion,
means for establishing a first communications link between the base station and all the mobile stations comprising the group,
means for monitoring the cell parameter and for detecting when the parameter meets a predetermined second criterion,
means for transmitting a handover signal when the cell parameter meets the second criterion,
and means for establishing a second communications link and severing the first communications link in response to the handover signal.
The base station may comprise a BTS as previously described.
The means for monitoring a characteristic of transmissions may be located in a mobile station and/or in a BSC, a BTS or an MSC.
The means for monitoring a cell parameter may be located in a BTS, a BSC or an MSC.
The means for clustering may be located at a BTS, a BSC or an MSC.
The cell parameter may be the loading of the cell, the amount of resource available, available capacity or interference levels for example. If, for instance, the cell becomes congested to a point where the loading reaches a pre-set threshold, then a group handover is triggered.
The characteristic of the monitored transmissions between the base station and the mobile station may be, for example, the strength of a transmitted or received signal, signal quality, quality of service, bit error rate or timing advance or can relate to whether the transmissions are speech or some other form of data such as facsimile for example.
Mobile stations may be grouped according to whether they lie within a particular region of the cell, the region being defined by signal power measurements, for example. Alternatively, those mobile stations which are transmitting and receiving speech rather than some other form of data are clustered into the same group.
Some or all of the mobile stations may effect a handover from the first communications link to the second, depending on network status and requirements.
The invention supports both intra-system group handover and inter-system group handover. For example, if the base station has been configured to support communication on two frequencies, the invention allows group handover from one frequency to the other. Further, the invention allows all mobile stations in a group to be handed over to another base station which may form part of the same or even a different network. Thus, the second communications link may be a new link with the base station on a different frequency or it may be a link with a different base station. For example, the invention can enable handovers between TDD and FDD (frequency division duplex) systems and TDD and GSM systems.
Advantageously, the invention allows group handover to occur with the minimum of signaling by virtue of the grouping procedure and the transmission of a single handover instruction to all mobile stations.
The invention can be utilised for simultaneously handing over a group of mobile stations which are in a common transportation platform such as a train.
The invention enables populations of MS""s to be exchanged between resources. This feature can be of a great benefit to network planners. For example, say an operator controls GSM and UMTS in FDD (frequency division duplex) mode. In certain circumstances, it may be beneficial to move, simultaneously, many mobile stations from one system to another in order to admit further calls. For instance if many voice callers are present on the UMTS FDD system, it can be useful to switch them over to GSM. The invention permits this with the minimum of signaling and time delay.
In one embodiment, the handover procedure is governed entirely by the base station which sends a handover signal to only those mobile stations in the clustered group.
In an alternative embodiment, all mobile stations within the cell receive a handover command from the base station and each mobile station plays a more active role by identifying whether or not it falls into the clustered group.
The clustered group may be confined between two threshold values. For example, the group could comprise those mobile stations whose received (uplink) power transmissions lie between two preset levels.